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Deposits - Narrative (Details) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Certificates of deposits denominations in dollar amount $ 250,000  
Maximum money market and demand deposit accounts purchasable 125,000,000.0  
Outstanding balance of ICS purchases $ 0 $ 0
Minimum    
Debt Instrument [Line Items]    
Certificate of deposits maturities period 3 months  
Certificate of deposits, fixed interest rates range (as a percent) 1.30% 1.00%
Maximum    
Debt Instrument [Line Items]    
Certificate of deposits maturities period 5 years  
Certificate of deposits, fixed interest rates range (as a percent) 3.45% 2.15%